In a 1952 paper by Paul, et al, the QIT and a slightly different device called the quadrupole mass spectrometer (QMS) were first disclosed. Mass spectrometers were known earlier but the QMS was the first mass spectrometer which did not require the use of a large magnet but used radio frequency fields instead for separation of ions of a sample, i.e., performing mass analysis. Massspectrometers are devices for making precise determinations of the constituents of a material by providing separations of all the different masses in a sample according to their mass (m) to charge (e) ratio (m/e). Mass spectrometers need to first disassociate/fragment a sample into charged atoms, i.e., ions, or molecularly bound group of atoms and then employ some mechanism for determining the M/e ratio of those fragments.
The QMS mechanism for separating ions relies on the fact that within a specifically shaped structure, radio frequency fields can be made to interact with an ion within the structure so that the resultant force on the ion is a restoring force which causes certain ions to oscillate about some referenced position. The QIT is capable of providing restoring forces on selected ions in three orthogonal directions. This is the reason that it is called a trap. Ions so trapped can be retained for relatively long periods of time which enables various operations and experiments on selected ions.
By changing one of the QIT parameters, it is possible to cause consecutive values of m/e of stored ions in the trap to become unstable and to pass those ions into a detector. The detected ion current signal intensity, as a function of the scan parameter, is the mass spectrum of the trapped ions.
Techniques are available to isolate an ion by scanning the QIT and to eject all ions except ions of a certain selected m/e value. If those isolate ions are considered a "parent", and they are further disassociated by some technique, "daughter" ions are formed which can be analyzed, or a single daughter ion can be isolated and further "daughters" obtained. This is known as MS/MS or MS.sup.n spectroscopy.
The preferred technique for further ion disassociation is a gentle ionization method called Collision Induced Disassociation (CID). The usual technique to obtain CID as described by Syka in U.S. Pat. No. 4,736,101 is to cause the ion to be excited at the secular frequency for the selected mass to increase the translational motion and decrease the mean time between collisions. According to the Syka technique, a signal at the secular frequency is applied to the end caps of the QIT. The kinetic motion energy is translated into internal energy on collision which results in gentle daughter ion fragmentation.
The Syka technique has a problem because it is extremely difficult to know the exact secular frequency required in advance to gently excite a particular ion. This is due to space charge effects in the trap relating to the number of ions and the molecular weight of the trapped ions and due to slight mechanical errors in the electrode shapes.
In the invention incorporated herein by reference, the inventors modulated the RF trapping field voltage at the same time that the "tickle" approximate secular frequency was supplied in order to provide sufficient frequency excitation coincident with the secular frequency to induce CID.
Another approach is to apply a continuum of CID frequencies to the QIT end caps to excite each generation of ions as disclosed by McLuckey, "Collisional Activation with Random Noise in Ion Trap," Anal. Chem. 64, 1992, 1455-1460. Typically, noise excitation is the broad band frequency source. The problem with this approach is that it causes the ions, both the parent and the daughter ions to disassociate without any control over the power absorbed by any particular ion.
Another broad band excitation technique is described by Yates, et at, at 39th MAS Conference Report on Mass Spectroscopy and Allied Topics, in a paper entitled "Resonant Excitation for GC/MS/MS in the 0IT via Frequency Assignment Prescans and Broadband Excitation", p. 132. This technique applied a 10 KHz band width described orally as a synthesized inverse FT time domain waveform to the QIT end caps so that the waveform has a frequency domain representation comprising a band of uniform intensity equally spaced frequencies up to .+-.5 KHz about a center frequency at the calculated theoretical secular frequency.
The difficulty with this Yates approach is that the noise amplitude and duration can be used to establish the fluence (power x time) for an ion of particular mass but with this technique the other ions cannot be optimized. Over excitation can cause ejection of the selected ion rather than disassociation. This ejection effect is amplified where ions are formed far from QIT center and absorb energy from noise immediately without being damped back to the QIT center.